Moisture-measuring system using microwave and nucleonic measurements

ABSTRACT

A system for the measurement of the percentage of moisture content of flowable bulk material online using microwave and nucleonic measurements, in which a microwave system has transmitting and receiving antennas and a readout system connected to the receiving antenna to measure the attenuation of the microwave signal including a third antenna to pick up scattered microwave energy to apply to the microwave readout a correction in accordance with the level of the scattered energy, the accuracy of readout being maintained by a PIN diode modulator in the output of the receiving antenna with a bias control loop for maintaining a constant output with the bias signal applied to the PIN diode representing moisture content of the material following correction in accordance with the scattered energy, and in which variations in mass are corrected by a nucleonic mass measuring gauge with its rays being aligned in intersecting relation with the microwave energy at the material being measured.

United States Patent Cornetet, Jr.

[54] MOISTURE-MEASURING SYSTEM USING MICROWAVE AND NUCLEONICMEASUREMENTS [72] inventor: Wendell H. Cornetet, .lr., Columbus, Ohio731 Assignee: Industrial Nucleonlca Corporation [22] Filed: Apr. 3, 1967[21] Appl. No.: 628,066

[ Feb. 22, 1972 Chip Weight and Moisture" in Paper Trade Journal. Feb.6, 1967. TS 1080. P26. pp. 48-49.

l-lewlett- Packard Journal, Vol. 16, No. 6, Feb. I965 pgs. 3 & 4.

Primary Examiner-Edward E. Kubasiewicz Attorney-Marechal, Biebel, Frenchand Bug and William T. Fryer, ll!

[ ABSTRACT A system for the measurement of the percentage of moisture plG01, 27/04 content of flowable bulk material online using microwave and5 Fi id fs h 324/58 58 250/83 3 D nucleonic measurements, in which amicrowave system has transmitting and receiving antennas and a readoutsystem con- 56] Relem c nected to the receiving antenna to measure theattenuation of the microwave signal including a third antenna to pick upscat- UNITED STATES PATENTS tered microwave energy to apply to themicrowave readout a correction in accordance with the level of thescattered enerl g g 2 gy, the accuracy of readout being maintained by aPIN diode 2888643 5/1959 S ay a 324/82 modulator in the output of thereceiving antenna with a bias 3068398 12]962 g 51 5 A control loop formaintaining a constant output with the bias 3265873 8/1966 e 5 3 Asignal applied to the PIN diode representing moisture content 29372805/960 Gina" 250/83 3 of the material following correction in accordancewith the 3278841 /1966 scattered energy, and in which variations in massare corrected by a nucleonic mass measuring gauge with its rays3,361,964 1/1968 Hanson et al ..324/6l being aligned in intersectingreafion with the microwave eneb OTHER PUBLICATIONS gy at the materialbeing measured.

Zehnder, C. B. Microwave- Gamma Ray Gauge Measures 9Claims, 2 DrawingFigures 62 DIFFERENTIAL 46 AMPLIFIER 40 t T OSCILLATOR AND VOL AGE POWERSUPPLIES 4i? CONTROL DETECTOR AND AMPLIFIER 6O 41 42 45 DIRECTIONALDETECTORI DIFFERENTIAL COUPLER No. 1 j AMPLIFIER 35 ADJUSTABLE LEVEL 343 6 ATTENUATOR SET MATERIAL 15 63 UNDER TEST RECEIVING 3a HORN ANTENNA5 52 7 PIN DIODE LEVEL I CONTROLLER 55\ THERM ELE TRIC TEMPERATSRECOMPARATOR AND CONTROL FOR o 2 DETECTOR BIAS CONTROL I DETECTOR No.2 ANDAMPLIFIER I 66 MASS 64 GUAGE INPUT Q PATENTEUFEBZZ I972 SHEET 1 OF 2 INVE/V T0 WENDELL H.CORNETET,JR.

M M g PATENTEDFEB22 I972 SHEET 2 OF 2 FIG -2 E2 DlFFERENTlAL 46AMPLIFIER r40 OSCILLATOR AND VOLTAGE POWER SUPPLIES A CONTROL DETECTORAND 7 AMPLIFIER 60 .SOLATOR 41 DIRETlDNAL DETEcT'oR DIFFERENTIAL COUPLERN0.1 AMPLIFIER l p f ADJUSTABLE M43 LEvEL ATTENUATOR I SET 48 RADIATING30 HORN ANTENNA MATERIAL r75 UNDER TEST N63 RECEIVING 33 HORN ANTENNA5'0 52 70 PIN DIODE LEvEL s CONTROLLER 55 THERNID'ELEDTRID 54 YTEMPERATURE COMPARATOR AND CONTROL FOR No 2 DETECTOR BIAS CONTROL -u---w DETECTOR No.2 AND AMPLIFIER 66 MASS GUAGE INPUT MOISTURE-MEASURINGSYSTEM IJSING MICROWAVE AND NUCLEONIC MEASUREMENTS BACKGROUND OF THEINVENTION The amount of water expressed inpercentage of weight of bulkmaterial, such as tobacco, cotton, flour, grain, and the like, has beena matter of interest for a considerable period of time, and varioussystems have been proposed, including microwavesystems in which theattenuation of a microwave signal is determined as a function ofmoisture content. It is known that there is a linear relation betweenelectromagnetic energy absorption, and water content, "which may beexpressed in grams of water per square centimeter of incidence face, formany inorganic materials. The slope'of the line representing thisrelationship varies with the wavelength of the electromagnetic energy,but it was not affectedappreciably by soluble salts. Also, attenuationdecreases with increasing wave length.

In the 0.8. pending application of Evans and Cornetet, Jr., Ser.No.'556,224, filed June 8, 1966, there is disclosed and claimedmicrowave systems for measuring the moisture content in a web or sheetof dielectric material, such as paper, but the techniques and apparatusdisclosed and claimed in that application do not necessarily lendthemselvesreadily to the measurement of moisture content in loose, bulkmaterials, such as flour, grain, tobacco and the like.

There are several advantages of using microwaves in measuring moisturecontent in flowable materials. No contact with material under test isnecessary. "Thus, continuous measurements are possible withoutcontamination or destruction of material. Also, measurements maytie-made through a relatively large area or sample of the material, andminimum control of the material is necessary as to density, arrangementon or in the.conveying apparatus,"or depth and width of the materialunder test.

SUMMARY or rm; INVENTION The moisture-measuring system of the presentinvention utilizesa microwave-signal-generatingsystem, which forms amicrowave signal at a substantially constant level, applied to atransmitting antenna. The antenna may be of the simple type such as anelectromagnetic horn. A receiving antenna is also employed to receivethe microwave energy transmitted by the transmitting antenna, whichenergy will be attenuated due in part to absorption bymoisture withinthe material.

The energy which is scattered by the material and therefore lost to thereeivingantenna, if not measured and accounted for, would be indicatedby the receiving system as moisture absorption. The effect of scattercannotreadily be accounted for as a'constant in flowable bulk materialdie to the differences'in scatter at the interface between the energyand the material caused by such randomly arranged material. Thus, theamount of scatter may vary with the type of material being tested andwith the density and arrangement of the material under observation. Inflowable bulk materials, it may be expected that scatter will vary withtime, Thus, this invention utilizes a second receiving antenna which ispositioned with respect to the material under test to receiv'e a portionof the scattered-energy therefrom, and the detected output of the secondantenna is applied as a correctiorito the readout.

It has been found that increased accuracy and reliability in readout canbe achieved by the use of a PIN diode modulator connected in shuntrelation to the receiving antenna so that a reasonably constant powerlevel to the output detector and amplifier can be maintained, In thismanner, the output detector and amplifier can be operated at a constantlevel, thus with a minimum-of error. PIN diode limiters have'a widedynamic range and are suitable for this purpose. A control loop isemployed for biasing the PIN diode modulator to niaintain the outputconstant, and the level of bias, following correction by the signal fromthe scatter-receiving antenna, may be calibrated directly in percentageof moisture content.

In systems'where the bulk of the massof material under measurement issubject to variations, a nucleonic type of mass measuring gauge isemployed with its radiation axis inclined to that of the microwave axisso as to intersect the bulk material at a common area. The detectedsignal from the radiation gauge, indicative of the mass of the materialunder test, may also be applied as a correction factor to the readout.

It is accordingly an important object of the present invention toprovide an online moisture-measuring system utilizing microwaves withcorrection for scatter of the microwave energy at its interface with thematerial under test.

Another important object of this invention is the provision, in amicrowave moisture measuring system, of a PIN diode limiter in a controlcircuit in which the output thereof is maintained constant and the levelof bias is utilized as an indication of moisture content.

A further object of this invention is the provision of a microwavesystem for the measurement of the moisture content online in flowablebulk materials in which a nucleonic gauge, such as one using an isotopesource, is positioned, with respect to the microwave system, to measurethe mass of the material at the same time that the material is beingsubject to microwave moisture measurement, to apply a mass correctionwhich is directly related to the moisture content being measured,without the necessity of further correction due to time delay or spacingbetween the mass measurement and the moisture measurement stations.

More specifically, it is an object of the present invention 'to providea moisture-measuring system, as outlined above, in which the nucleonicgauge ray is directed through flowable bulk material, online, in angularintersecting relation to the microwave energy at such material, togetherwith a readout system in which the affects of variations in the mass ofthe material under test may be eliminated providing a direct indicationof the percentage of moisture content.

These and other objects and advantages of the present in vention will beapparent. from the following description, the accompanying drawings andthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective and somewhatdiagrammatic view of a moisture measuring system constructed accordingtothis invention; and

FIG. 2 is a block diagram of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT Referring .to the figures of thedrawing which illustrate a preferred embodiment of the invention, anonmetalic conveyor is illustrated generally at X0 in FIG. 1 fortransporting bulk material 15, the moisture content of which is to bemeasured by this system. The carrier, of course, may be a closedconduit. The material 15 maybe, and usually is, a flowable bulk materialsuch as grain, tobacco, or other loosely packed, moisture-containingproduct which may be carried on a conveyor 1001' otherwise transported.

The material 15 is carried by the conveyor 10 through a measuringstation at which the moisture content of the material, as it passesthrough this station, will be measured in an online condition. Themeasuring station is indicated generally at 16 in FIG. 1 and is definedas the region of intersection of the mass-measuring ray and themicrowave energy. Thus, as shown in FIG. 1, means for constantlymeasuring the mass of the material 15, the moisture content of which isto be measured, includes a radiation source 20 positioned to directradiation therefrom, as generally indicated by the axis 21, through thematerial 15, and is collected bya detector 22. The detector 22 functionsas a'high resistance whose impedance varies as an inverse function ofthe intensity of the received radiation, and could be an ionizationchamber, or other known means, dependent upon the source being used.

The mass responsive gauge consisting of the source 20 and detector 22 ispreferably of the. penetrative radiation type wherein a source of beta,gamma, bremsstrahlung or other type radiation is used to irradiate thematerial to be measured and a radiation detector such as a scintillationdetector or an ionization chamber is used to determine the amount ofradiation passing through the material. The choice of radiation dependson the density of the material being measured. Whereas a useful detectorresponse results from the use of beta or bremsstrahlung radiation forlightweight materials, there would be substantially no absorption of amore penetrating radiation such as gamma. Alternatively, it is desirableto use a gamma source when one measures a relatively dense material.Strontium 90 and Krypton 85 are examples of beta emitters and Americium241 and Cesium I37 are suitable gamma emitters. Irrespective of theweight range of the material, the radiation detector signal is combinedwith the microwave detector signal to provide an output signal directlyproportional to percent moisture.

The output of the detector 22 may be applied to a suitable computerbalancer and a standardizing circuit, the output providing a voltageindicative of the mass of the material generally at the zone or region16 of intersection, The axis 21 of the radiation gauge is preferablyinclined slightly to the perpendicular with respect to the conveyor 10,and may, as shown in FIG. 1, be inclined slightly in the direction oftravel and transverse to the direction of travel in order to providephysical space for the positioning of the microwave transmitting andreceiving components.

For highest accuracy, it is desirable to maintain a fairly constant massof material 15 flowing through or in the measurement station 16, toreduce error due to nonlinearity of the measurement system. However, byreason of the employment of the radiation gauge, it is not necessary tomaintain the mass ofsuch material precisely constant.

The microwave system includes a transmitting antenna which is preferablyof the simple electromagnetic horn type for directing radiant energyalong an inclined axis 31 through the material 15 at the zone 16 to afirst microwave receiving antenna 32. Thus, the antenna 32 is positionedrelative to the transmitting antenna 30 to receive the attenuatedtransmitted microwave energy passing through the material 15 and thesupport or conveyor 10. As shown in FIG. 1, the axis 31 ofthemicrowave-measuring system is inclined with respect to that of theradiation gauge for the purpose of providing space for the physicalplacement of the parts of the system. Accordingly, the axis 31 may alsobe inclined slightly from the perpendicular with respect to thedirection of travel and transversely of the direction of travel, and theantennas 30 and 32 are so positioned that the transmitted microwaveenergy intersects the material 15 substantially at the same area as thatintersected by the radiation-measuring equipment, defining the zone 16.

The effect of energy scatter at the interface between the transmittingantenna energy and the test material, in many cases, may be significant.Furthermore, the scatter patterns and amount may vary during theoperation of the instrument, due to variations in the random pattern ofthe material 15. The energy lost by scattering at such interface wouldappear at the output as added attenuation in the absence of means fordetecting, measuring scatter and correcting the output in accordancewith the measured level of scattered energy. For this purpose, a thirdmicrowave-receiving antenna 35 is employed by an angle to the axis 31,preferably with its axis 36 forming the same angle with the base orconveyor 10 as that formed by the axis 31, and on the same side of thematerial 15, in a region of maximum scattered energy. The actualpositioning of the antenna 35 is not critical, and for any giveninstallation a position may be found at which such scattered energy maybe measured corresponding roughly along a line of maximum reflection.

A typical circuit diagram for the apparatus of FIG. 1 is shown in FIG.2. The circuit of FIG. 2 includes means for applying a substantiallyconstant microwave energy to the transmitting antenna 30. This mayinclude an oscillator 40. The frequency and output of the oscillator 40will depend upon the type and amount of material under test. Forexample, a

smaller amount ofmaterial may require or necessitate a higher frequency,while lower output may be satisfactory. Typical frequencies may lie inthe range of 6 to 16 GHz. An oscillator 40 output level of approximately200 milliwatts is satisfactory, in most cases, again depending upon theamount of material under test.

The output of the oscillator is applied through an isolator 41 and adirection coupler 42, through an adjustable attenuator 43 to the antenna30. A portion of the output is taken off by a 'detector 44 and appliedto a differential amplifier 45 and a voltage control circuit 46, theoutput of which is applied to a line 47 to control the level of theoscillator 40. The output of the oscillator 40 may thus be variedaccording to the setting of a level set control 48 having an input intothe amplifier 45. In this manner, a substantially constant and regulatedoutput level is applied to the antenna 30.

The adjustable attenuator 43 is initially set for the necessary powerlevel for the material under test. If temperature causes any appreciablevariation in this attenuator, this can be included in the level controlloop. The level set potentiometer 48 permits further control over theoutput power and allows adjustment of the dynamic range of control. Anychange in detected signal at the detector 44 is compared with the levelset voltage by the amplifier 45 and causes a change in oscillator 40output to correct for the detected change.

Circuit means connected to the receiving antenna 32 for providing anelectric signal proportional to the energy received therefrom includes aPIN diode modulator 50 forming a part ofa constant output level controlloop. The diode 50 is connected in shunt across the output of thereceiving antenna 32 and forms an output modulator ofwide dynamic range.

The diode modulator 50 includes PIN diodes consisting of P-typematerial, an intrinsic layer of material, and N-typc material, and themodulator 50 may contain a number of such diodes mounted as shuntelements mounted across the transmission line. A suitable PIN diodemodulator for the modulator 50 may be the type 8734A of Hewlitt-PackardCorporation, 1501 Page Mill Road, Palo Alto, California. When DC forwardbias is applied, such as by the input line 52, modulator resistance goesdown. Thus, the diodes in the modulator 50 act as a variable impedancedevice shunting the transmission line, with their impedance and degreeof attenuation being a function of the modulating signal applied on line52.

The output of the modulator 50 is relatively constant and is applied toa detector and amplifier 54 and then to a comparator and bias controlcircuit 55. The comparator circuit 55 has an input on line 56 from thedetector 44 forming a reference level in accordance with that of thetransmitted energy, and provides a modulating bias signal on line 52 tothe modulator 50 to maintain a constant input into the detector andamplifier 54 over variations in the microwave signal received by theantenna 32. Accordingly, the bias signal 52 varies as a function ofmoisture content and is more accurately a direct indication of totalattenuation. Also, the level set 48 may be adjusted to obtain a desiredoutput. Adding test material 15 attenuates the received signal, thedetector output decreases, and the comparator and bias control 55 causesa change in bias on the PIN modulator 50 so that the detected signal atthe detector 54 is brought to the original level. Thus this change inbias level represents moisture content of the sample under test. It isthen corrected by the changes in scattered energy at the amplifier 64,and is further modified by the input from the radiation gauge at thedifferential amplifier 65, to provide a signal of the percent ofmoisture in the sample under test.

It is accordingly seen that this invention provides a relativelylow-cost and accurate microwave system for the online measurement of thepercent of moisture in flowable bulk materials. The employment of theconstant level output circuit and the PIN diode modulators provides ahighly accurate and reliable readout. The employment of the scatterantenna 35 permits correlation and correction to be made for the loss ofsignal due to the scatter of microwave energy from the interface of thematerial 35 with the transmitted microwave energy. In the absence ofsuch correction as applied by this invention, this loss of energy wouldbe measured as attenuation at the receiving antenna and would result inan erroneous indication of percent of moisture.

Means for providing a signal proportional to the scattered energyincludes a detector and amplifier 60 connected to receive the signalfrom the scatter antenna 35 and apply it to a differential amplifier 62,which also has a reference level input connected to the output of thedetector 44. The output of the amplifier 62 is a signal proportional tothe amount of energy scattered by the material 15. This signal isapplied from the amplifier 62 on lead Field 63 to a differential ratioamplifier 64 which combines the bias signal of the modulator 50 with thesignal proportional to the scatter from the antenna 35.

The output of the differential amplifier 64 may be applied to a furthersuch differential amplifier 65 intowhich the output of the massmeasuring gauge is applied on a lead 66, to make correction forvariations in mass. The combined output may thus be applied to a readoutmeans 68, providing a readout of the percentage of moisture. Thus, whileit is not necessary to control the exact amount of material passingthrough the moisture area, large changes in mass may call for a ratherwide dynamic range in the measuring system. The output can be appliedfor direct reading to a meter 68, or may be used for automatic controlof moisture in a closed-loop control.

If the detector and amplifier 54 are unduly temperature sensitive,correction may be applied by a thermoelectric control detector 70 toprovide a controlled environment by correcting for changes in ambienttemperature.

In the operation of the system, with no material between the radiatingantenna 30 and the receiving antenna 32, the comparator and bias controlunit 55 is adjusted for reference output at zero percent moisture.

While the form of apparatus herein described constitutes a preferredembodiment of the invention, it is to be understood that the inventionis not limited to this precise form of apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

What is claimed is:

1. A microwave moisture-measuring system for online measurement ofmoisture in fiowable bulk material, comprising means defining ameasuring station through which said material is moved for measurement,a nucleonic ray type of massmeasuring instrument at said station havingits measuring ray positioned to intersect said material at said stationand providing a signal proportional to the mass of such material, asource of microwave energy, a microwave-transmitting antenna connectedto said source and positioned to direct microwave energy through saidmaterial at said station so that the material at said station is subjectto nucleonic and microwave measurement, a receiving antenna positionedto receive energy from said transmitting antenna attenuated by moisturein the material at said station, first circuit means connected to saidreceiving antenna including a PIN diode modulator connected in shuntrelation across the output of said receiving antenna providing anelectric signal proportional to the energy received therefromrepresenting the loss of microwave signal due to absorption of moistureby said material, detector and amplifier means connected to receive saidelectric signal, comparator means connected to receive the output ofsaid detector and amplifier means and providing a bias signal to saidmodulator for maintaining the output thereof substantially constant overvariations in moisture content of said material, and signal combiningmeans combining said mass proportional signal and the output of saiddetector and amplifier means providing a direct indication of percentageof moisture content of the material at said station.

2. A microwave moisture-measuring system for online measurement ofmoisture in flowable bulk material, comprising means defining ameasuring station through which said material is moved for measurement,a nucleonic ray type of mass measuring instrument at said station havingits measuring ray positioned to intersect said material at said stationand providing a signal proportional to the mass of such material, asource of microwave energy, a microwave transmitting antenna connectedto said source and positioned to direct microwave energy through saidmaterial at said station, a first receiving antenna positioned toreceive transmitting energy from said transmitting antenna passingthrough said material, a second receiving antenna positioned to receivemicrowave energy scattered from said material, first circuit meansconnected to said first receiving antenna providing an electric signalproportional to the energy received therefrom, second circuit meansconnected to said second antenna providing an electrical signalproportional to the scattered microwave energy first, signal combiningmeans for combining the signals of said first and second circuit meansproviding a corrected signal of microwave attenuation representing theloss of microwave signal due to absorption of moisture by said material,and second signal combining means combining said mass proportionalsignal and said corrected signal providing a direct indication ofpercentage of moisture content of said material at said station.

3. The system of claim 2 in which said material at said station movesalong. a generally rectilinear path, and in which said nucleonicmass-measuring instrument and said microwave antennas are positioned sothat the nucleonic ray and radiated microwave energy are directed alongpaths which are mutually inclined to each other and which mutuallyintersect each other and said material at said station to subject saidmaterial to simultaneous microwave and nucleonic measurement.

4. The system of claim 2 in which said first circuit means includes aPIN diode modulator connected in shunt relation across the output ofsaid first receiving antenna, detector and amplifier means connected toreceive the signal passes through said modulator, and comparator meansconnected to receive the output of said detector and amplifier means andproviding a bias signal to said modulator for maintaining the outputthereof substantially constant over variations in moisture content ofsaid material, and in which the output of said first circuit meanscomprises a signal proportional to said bias signal.

5. A microwave system for measuring the moisture content in a sample ofdielectric material, comprising means generating a microwave signal at asubstantially constant level, a transmitting antenna positioned todirect microwave energy from said generating means through said sample,a receiving antenna positioned with respect to said material to receivea signal therethrough from said transmitting antenna which is attenuatedby absorption due to moisture in said sample, a PIN diode modulatorconnected in shunt relation across the output of said receiving antenna,detector and amplifier means connected to receive the signal passedthrough said modulator, comparator means referenced to said microwavegenerator means connected to receive the output of said amplifier meansand providing a bias signal to said modulator for maintaining the outputof said modulator substantially constant over variations in saidmoisture content, and read out means responsive to the level of biassignal as a function of the moisture content of said material.

6. The system of claim 5 further comprising a second receiving antennapositioned with respect to said material for receiving microwave energyscattered therefrom, detector and amplifier means connected to saidsecond antenna and having its output connected to said readout means forcorrecting the readout thereof in accordance with the level of scatteredmicrowave energy.

7. The system of claim 5 further comprising nucleonic measuring meanspositioned to direct radiation in intersecting relation to saidmicrowave energy at said material, means connected to said measuringmeans providing a signal responsive to the mass of said material, anddifference amplifier means at said readout'means connected to receivesaid mass signal and said bias signal for correcting said readout inaccordance with variations in mass of said material.

8. The system of claim 7 in which the axis of said radiation and thepath of said microwave energy are mutually offset to intersect saidmaterial at somewhat different angles providing for the adjacentplacement of the said antennas and said nucleonic measuring means.

9. A microwave moisture-measuring system for online measurement ofmoisture in fiowable bulk material, comprising means defining ameasuring station in which said material is positioned for measurement,a nucleonic ray type of mass measuring instrument at said station havingits measuring ray positioned to intersect said material at said stationand providing a first signal proportional to the mass of such material,a source of microwave energy, a microwave transmitting antenna connectedto said source and positioned to direct microwave energy through saidmaterial at said station so that the same material at said station issubject to nucleonic and -microwave measurement, first antenna means forreceiving energy from said transmitting antenna attenuated by moisturein the material at said station to provide a second signal proportionalto the moisture in said material, means for combining said first andsecond signals to provide an indication of percentage of moisturecontent of the material at said station, and second receiving antennameans positioned adjacent said transmitting antenna to receive microwaveenergy scattered by said material including conduit means providing anelectrical signal proportional to the microwave energy scattered by saidmaterial and being connected to said signal combining means providing acorrected signal of microwave attenuation representing the true loss ofmicrowave signal due to absorption thereof by said material.

1. A microwave moisture-measuring system for online measurement ofmoisture in flowable bulk material, comprising means defining ameasuring station through which said material is moved for measurement,a nucleonic ray type of mass-measuring instrument at said station havingits measuring ray positioned to intersect said material at said stationand providing a signal proportional to the mass of such material, asource of microwave energy, a microwave-transmitting antenna connectedto said source and positioned to direct microwave energy through saidmaterial at said station so that the material at said station is subjectto nucleonic and microwave measurement, a receiving antenna positionedto receive energy from said transmitting antenna attenuated by moisturein the material at said station, first circuit means connected to saidreceiving antenna including a PIN diode modulator connected in shuntrelation across the output of said receiving antenna providing anelectric signal proportional to the energy received therefromrepresenting the loss of microwave signal due to absorption of moistureby said material, detector and amplifier means connected to receive saidelectric signal, comparator means connected to receive the output ofsaid detector and amplifier means and providing a bias signal to saidmodulator for maintaining the output thereof substantially constant overvariations in moisture content of said material, and signal combiningmeans combining said mass proportional signal and the output of saiddetector and amplifier means providing a direct indication of percentageof moisture content of the material at said station.
 2. A microwavemoisture-measuring system for online measurement of moisture in flowablebulk material, comprising means defining a measuring station throughwhich said material is moved for measurement, a nucleonic ray type ofmass measuring instrument at said station having its measuring raypositioned to intersect said material at said station and providing asignal proportional to the mass of such material, a source of microwaveenergy, a microwave transmitting antenna connected to said source andpositioned to direct microwave energy through said material at saidstation, a first receiving antenna positioned to receive transmittingenergy from said transmitting antenna passing through said material, asecond receiving antenna positioned to receive microwave energyscattered from said material, first circuit means connected to saidfirst receiving antenna providing an electric signal proportional to theenergy received therefrom, second circuit means connected to said secondantenna providing an electrical signal proportional to the scatteredmicrowave energy first, signal combining means for combining the signalsof said first and second circuit means providing a corrected signal ofmicrowave attenuAtion representing the loss of microwave signal due toabsorption of moisture by said material, and second signal combiningmeans combining said mass proportional signal and said corrected signalproviding a direct indication of percentage of moisture content of saidmaterial at said station.
 3. The system of claim 2 in which saidmaterial at said station moves along a generally rectilinear path, andin which said nucleonic mass-measuring instrument and said microwaveantennas are positioned so that the nucleonic ray and radiated microwaveenergy are directed along paths which are mutually inclined to eachother and which mutually intersect each other and said material at saidstation to subject said material to simultaneous microwave and nucleonicmeasurement.
 4. The system of claim 2 in which said first circuit meansincludes a PIN diode modulator connected in shunt relation across theoutput of said first receiving antenna, detector and amplifier meansconnected to receive the signal passes through said modulator, andcomparator means connected to receive the output of said detector andamplifier means and providing a bias signal to said modulator formaintaining the output thereof substantially constant over variations inmoisture content of said material, and in which the output of said firstcircuit means comprises a signal proportional to said bias signal.
 5. Amicrowave system for measuring the moisture content in a sample ofdielectric material, comprising means generating a microwave signal at asubstantially constant level, a transmitting antenna positioned todirect microwave energy from said generating means through said sample,a receiving antenna positioned with respect to said material to receivea signal therethrough from said transmitting antenna which is attenuatedby absorption due to moisture in said sample, a PIN diode modulatorconnected in shunt relation across the output of said receiving antenna,detector and amplifier means connected to receive the signal passedthrough said modulator, comparator means referenced to said microwavegenerator means connected to receive the output of said amplifier meansand providing a bias signal to said modulator for maintaining the outputof said modulator substantially constant over variations in saidmoisture content, and read out means responsive to the level of biassignal as a function of the moisture content of said material.
 6. Thesystem of claim 5 further comprising a second receiving antennapositioned with respect to said material for receiving microwave energyscattered therefrom, detector and amplifier means connected to saidsecond antenna and having its output connected to said readout means forcorrecting the readout thereof in accordance with the level of scatteredmicrowave energy.
 7. The system of claim 5 further comprising nucleonicmeasuring means positioned to direct radiation in intersecting relationto said microwave energy at said material, means connected to saidmeasuring means providing a signal responsive to the mass of saidmaterial, and difference amplifier means at said readout means connectedto receive said mass signal and said bias signal for correcting saidreadout in accordance with variations in mass of said material.
 8. Thesystem of claim 7 in which the axis of said radiation and the path ofsaid microwave energy are mutually offset to intersect said material atsomewhat different angles providing for the adjacent placement of thesaid antennas and said nucleonic measuring means.
 9. A microwavemoisture-measuring system for online measurement of moisture in flowablebulk material, comprising means defining a measuring station in whichsaid material is positioned for measurement, a nucleonic ray type ofmass measuring instrument at said station having its measuring raypositioned to intersect said material at said station and providing afirst signal proportional to the mass of such material, a source ofmicrowave energy, a microwave traNsmitting antenna connected to saidsource and positioned to direct microwave energy through said materialat said station so that the same material at said station is subject tonucleonic and microwave measurement, first antenna means for receivingenergy from said transmitting antenna attenuated by moisture in thematerial at said station to provide a second signal proportional to themoisture in said material, means for combining said first and secondsignals to provide an indication of percentage of moisture content ofthe material at said station, and second receiving antenna meanspositioned adjacent said transmitting antenna to receive microwaveenergy scattered by said material including conduit means providing anelectrical signal proportional to the microwave energy scattered by saidmaterial and being connected to said signal combining means providing acorrected signal of microwave attenuation representing the true loss ofmicrowave signal due to absorption thereof by said material.